fix(infra): disable Next image disk cache for read-only Nix store

Next.js image optimizer mkdirs <distDir>/cache/images on first
optimized-image request. In the Nix store that path is read-only, so
every image request throws EACCES unhandledRejection. images.maximumDiskCacheSize=0
plus experimental.isrFlushToDisk=false keep the optimizer in-memory only.

next.config.ts

@@ -5,6 +5,16 @@ const nextConfig: NextConfig = {
   turbopack: {
     root: process.cwd(),
   },
+  // Nix store is read-only at runtime; Next.js tries to mkdir
+  // <distDir>/cache/images for the on-disk image optimizer LRU and
+  // crashes with EACCES unhandledRejection on every optimized image
+  // request. Disable the disk cache (and disable ISR flush which gates it).
+  images: {
+    maximumDiskCacheSize: 0,
+  },
+  experimental: {
+    isrFlushToDisk: false,
+  },
 };
 
 export default nextConfig;